1. Field
The present disclosure relates to a zoom lens which is suitable for use in an image projecting apparatus (projector) configured to project, for example, an image in an enlarged scale.
2. Description of Related Art
In the related art, various image projecting apparatuses (projectors) exist. Known projection apparatuses are generally configured to use an image display device such as liquid crystal display and project an image on the basis of the image display device on a screen surface in an enlarged scale by a projection optical system. To that end, the projector is desired to project a bright projection image in order to ensure appropriate visibility when the image is projected in an enlarged scale. The projection optical system used in the projector is desirably a zoom lens capable of projecting the image in various projection magnifications. Depending on the environment where the projector is installed, the projection optical system requires certain specific installation properties. However, the zoom lens for the projector has a problem in that the brightness of the projected image changes at the time of zooming when an F number (aperture number) changes, which is not desirable in terms of viewability. Therefore, various attempts have been made to obtain a projection zoom lens free of such shortcomings.
U.S. Pat. No. 7,605,985 and U.S. Pat. No. 8,390,935 disclose the zoom lenses configured to maintain the brightness of a projected image by maintaining the F number over the entire zoom range of the zoom lens for projectors.
A projection zoom lens disclosed in U.S. Pat. No. 7,605,985 includes first to sixth lens units having refractive powers of negative, positive, positive, positive, positive, and positive arranged in this order from the enlarging side, and an aperture having a fixed or variable aperture diameter and moving independently at the time of zooming provided between the fourth lens unit and the fifth lens unit. In this configuration, it is believed that the F number is kept constant over the entire zoom range.
A projection zoom lens disclosed in U.S. Pat. No. 8,390,935 includes first to sixth lens units having refractive powers of negative, positive, positive, negative, positive, and positive arranged in this order from the enlarging side, and an aperture having a mask moving integrally with the second lens unit at the time of zooming on the image side of the second lens unit. The projection zoom lens disclosed in U.S. Pat. No. 8,390,935 also includes an auxiliary stop having a fixed aperture at the time of zooming on the side of the object of the sixth lens unit. In this configuration, the F number is kept constant over the entire zoom range.
The zoom lens used in the projector is required to provide high-quality projected images, and be capable of projecting at a constant brightness with various enlargement magnifications with a smaller change in F number at the time of zooming when projecting the images. In order to provide high-quality projected images, and keep the brightness of the projected image constant at the time of zooming, that is, in order to keep the F number constant over the entire zoom range, adequate setting of a lens configuration such as zoom types or positions of the aperture stop is important.
In the projection zoom lens disclosed in U.S. Pat. No. 7,605,985, a configuration in which a change in F number is small at the time of changing the power by moving the aperture stop at the time of zooming. However, there is a problem that excessive light at the wide angle end cannot be cut and hence high quality image projection cannot be realized along the entire zoom range. In the projection zoom lens disclosed in U.S. Pat. No. 8,390,935, unnecessary light is cut by providing the auxiliary stop. However, since the aperture stop is fixed on a reduction conjugate side, there is a tendency for various aberrations to significantly vary from one zoom position to another, and a change in optical performance caused by zooming is increased.